Infrared detecting devices are specially used in different electrical products which efficiently save energy while detecting a human body.
A prior art device described Japanese Patent Publication No. 11-83624 is comprised of a pyroelectric element, an I/V conversion circuit, a voltage amplification circuit, a detection circuit and an output circuit. The pyroelectric element is operable to generate a micro current signal in response to variation of incoming infrared radiation from detection area through condenser lens or the like. The I/V conversion circuit converts the current signal from the pyroelectric element into a voltage signal. The voltage amplification circuit selectively amplifies components with prescribed frequencies of the voltage signal to issue a components-amplified voltage. The detection circuit is comprised of, for example, a window comparator and provides a comparison between the components-amplified voltage and a prescribed detection threshold voltage to issue a detection signal of the infrared radiation. The detection threshold voltage is a window threshold with a higher threshold voltage and a lower threshold voltage. The output circuit is comprised of, for example, level shift circuit or the like and issues an output signal in response to the detection signal. This infrared detecting device issues the output signal for representing detection of the infrared radiation (e.g., human body) when the components-amplified voltage is less or more than the window threshold range. The device also issues the output signal for representing non-detection of the infrared radiation when the components-amplified voltage is converged within the window threshold range.
A prior art device described Japanese Patent Publication No. 2002-156281 comprises a battery as a power source in addition to the pyroelectric element, the I/V conversion circuit, the voltage amplification circuit, the detection circuit and the output circuit. This infrared detecting device provides a rated current as a drive current to I/V conversion circuit, the voltage amplification circuit, the detection circuit and the output circuit when voltage of the I/V conversion circuit exceeds a prescribed voltage. When the voltage of the I/V conversion circuit is equal to or lower than the prescribed voltage, the device provides a smaller current than the rated current as the drive current to those circuits. In this case, current consumption (drive current) can be reduced.
However, in the construction that reduces current consumption as the device does, there is a problem that the performance or behavior of the circuits become unstable. Because the circuits will easily suffer dispersion of parts-performance such as threshold of transistors, resistance and capacitance, or dispersion of current consumption caused by temperature characteristics. There is also a trend that indicates greater dispersion of the current consumption in case the drive current to the circuits is much reduced to the level such as, for example, 10 s nA below a few μA. Considering these issues, since there is a need of design with sufficient allowance for power voltage, temperature characteristics, dispersion of parts-performance or the like in order to provide infrared detecting devices with low consumption, it is difficult to sufficiently save energy of the device.